Systems for protecting premises or areas against improper intrusion have become widely used, particularly those which more or less continually monitor the premises or area from a remote location. The latter typically includes a police or central station which is tied to the monitored premises or area, a bank, for instance, by means of electrical cables such as a pair of telephone lines. Signals are sent from the central station to "interrogate" the premises or area which in turn sends back an appropriate signal to indicate its status. In the event trouble arises somewhere in the system or any tampering or improper intrusion should occur at the protected premises or area, a failure or change of the return signal to the central station activates a trouble indicator or an alarm.
To be fully adequate, the system must not only monitor the premises or area but itself as well. This is because, besides warning of trouble or failure of the system, it must also guard against attempts to subvert it by inducing false signals that all is well when in fact it is not. Subversion can be accomplished, for instance, by monitoring the connecting lines to note and record the nature of the response of the premises or area when interrogated and no trouble or alarm situation exists. A false signal of well being can then be insinuated into the lines in response to each interrogation and the connecting lines cut or the system at the premises or area otherwise disabled. Various methods to secure such systems against subversion are well known. Some include sending a mixture of signals to the premises or area, only some of which actually provide the interrogation while the remainder are decoys, the former being hidden amid the welter of the latter. Others interrogate only at random intervals, thus making it difficult if not impossible to provide a false return signal at the necessary times. Still others provide line security only as a manually instituted adjunct to the main system rather than as an innate, automatic part of it. The nature of the interrogation and response signals also varies, pulses being used in some cases and tones in others, though the former seem to predominate. Trouble and alarm conditions are often indicated by voltage or impedance changes on the connecting lines or by reversal of their polarity in the case of a DC line system. Many systems tend to be rather if not inordinately complex, particularly those in which signals to and from several premises at different locations are issued and monitored from a common complex of apparatus at the central station rather than by a separate or discrete installation for each location.
It is the primary object of the present invention, therefore, to provide a system of the nature and for the purposes described which is simpler and more reliable than many while at the same time providing complete security both for the premises or area to be protected as well as for the lines interconnecting the latter with the central station.